British police acknowledged Saturday they could have done more to prevent three teenage girls from leaving their families to join the Islamic State in Syria.

Police questioned all three girls in December while investigating the disappearance of another girl from their school, a friend of theirs who had already gone to Syria.

The girls' families expressed anger that police did not inform them directly about their children's involvement in the investigation.

London's Metropolitan Police Service, commonly known as Scotland Yard, issued a statement saying that in hindsight, letters requesting permission to speak to the teens could have been delivered directly to their parents. Instead, investigators gave the letters to the girls to take home, and they never showed them.

The teens — Kadiza Sultana, 16, and Amira Abase and Shamima Begum, both 15 — disappeared from their homes in February, flying from London to Istanbul. They are now believed to be in Syria.

"There was nothing to suggest at the time that the girls themselves were at risk," said Scotland Yard in its statement. "We continue to liaise with the school and local education authority in connection with this ongoing investigation."

But family members say they would have been able to intervene if they had been aware of the situation with the girls' friend, and that they could have stopped them from leaving Britain.

Turkish officials have also criticized the response by British authorities, saying Britain took too long to notify them about the girls.

Thousands of foreigners from more than 80 countries have joined the ranks of the Islamic State and other radical groups in Syria and Iraq, many crossing through Turkey. Security officials estimate 500 or more people from Britain alone have traveled to Syria to join the conflict.

After the girls' disappearance, British Prime Minister David Cameron called on Internet companies to do more to address online extremism, saying the three girls appeared to have been radicalized while at home "in their bedrooms."